1. Field of the Invention
This invention relates to antenna systems in general and in particular to spiral antennas and feed systems. More particularly, this invention relates to antennas which provide a unidirectional, broadband, circularly polarized radiation beam centered on the spiral axis.
2. Description of the Prior Art
It is well understood that if the dimensions of an antenna are doubled and at the same time the wavelength is doubled, the performance of a log spiral antenna remains the same. More specifically, the impedance, polarization and pattern are invariant to a change of scale that is in proportion to the change in wavelength, provided the antenna is made of nearly perfect conductors and dielectrics. Therefore, if an antenna's shape is entirely determined by angles, the performance is independent of frequency, for such a shape would be invariant to a change of scale. In addition to this angle principle, it is necessary that the shape also fit a truncation principle.
The foregoing led to the development of conical and planar sheet equiangular spiral antennas which fit both the angle and truncation principles. However, frequency independent operation is actually observed only over a limited band. The low frequency limit is set by the maximum dimension, and the high frequency limit is set by how precisely the input terminal region balanced connector (the balun) can be constructed.
To illustrate the general approach, we consider all plane curves which remain essentially the same when scaled to a different unit of length. Such curves are used to determine the shape of a plane sheet antenna by taking the input terminals at the common point of intersection of two curves. It follows then that the antenna is unchanged when scaled to a different wavelength, provided the terminals stay fixed when the scale is changed. The fact that a typical curve remains essentially unchanged by a change of scale implies that the new curve can be made to coincide with the old curve by translation and rotation. Since a translation is eliminated by the requirement that the common point remain fixed, the problem is to determine all curves such that a change of scale is equivalent to a rotation. Stated symbolically: EQU K r(.phi.)=r(.phi.+C) (1)
where r (.phi.) denotes the radius r as a function of the polar angle .phi., K is the scale change, and C is the angle of rotation to which it is equivalent. Thus K depends on C, but K and C are independent of .phi. and r. Therefore: ##EQU1## However, ##EQU2## Therefore: ##EQU3## or EQU dr/d.phi.=ar (6)
where a is inependent of .phi. and is defined as: EQU a=(1/K)(dK/dC) (7)
But from (6):
.intg.(dr/d.phi.)=.intg.ar (8) EQU r=r.sub.o e.sup.a.phi. ( 9)
r.sub.o being a constant EQU Let r.sub.o =e.sup.a.phi..sbsp.o (10)
.phi..sub.o being a constant
Then from (9): EQU r=e.sup.a.phi..sbsp.o .multidot.e.sup.a.phi. (11)
or EQU r=e.sup.a(.phi.+.phi..sbsp.o.sup.)
Taking the natural log of both sides we get: EQU .phi.+.phi..sub.o =(1/a) 1nr (12)
The derived formula for an equiangular spiral, so indicates that the shape of all plane sheet frequency independent antennas must be defined by equiangular spirals.
While antennas with log spirals (r=e.sup.a.phi.) are frequency independent, it is also well known that Archimedean spirals (r=a.phi.) are well behaved broadband antennas. Furthermore, two arm Archimedean spirals over moderate bandwidths often out perform log spirals of comparable dimensions because more spiral turns can usually be incorporated with the Archimedean than the log spiral. Of course, while the log and Archimedean spirals are unique classical spirals of different growth rates, there is as described in the present invention an infinite class of spirals in addition to those two prior art examples, which we simply call general growth rate spirals (r=f(.phi.)).
Two dimensional antennas made out of a sheet of metal, or as in the present design by masking two thin matching copper spirals, one on each side of a non-conductive substrate, can be fed successfully by a coaxial line; however, a mode converter from the coaxial to biaxial geometry is needed (more commonly known as a balun).
Many different baluns have been invented, only some of which are in fact true mode converters, and fewer still that are frequency independent.
One such frequency independent type is simply a gradual transition from coaxial to biaxial cross-section, which clearly must accomplish the mode conversion if the transition is spread out over a sufficient number of wavelengths.
In prior art, the outer conductor of the coaxial line is gradually cut away so that after some distance less than half of it remains. At this point the resemblance to a pair of wires is clear, and a smooth deformation to the biaxial geometry easily follows; however, at microwave frequencies the cross-section becomes so small that the necessary precision of construction becoms very hard to obtain. Therefore, a different version was developed by etching copper from both sides of a thin dielectric sheet. The outer conductor of the coaxial line fits into a hole in the lower plate and the inner conductor is connected to the upper plate.
In the prior art, this connection was made orthogonally through the center of the spiral antenna; however such perpendicular mounting carried with it substantial limitations with respect to mounting or integrating the antenna within a system and further could induce coupling error at high frequency.
Other prior art provided for an antenna fed by a balanced line (coax) brought into the center point aong the axis; i.e. using a coaxial cable embedded into one arm of the spiral. For symmetry a dummy cable was sometimes soldered to the other arm of the spiral in a similar configuration.
Limitations of these prior art devices led to the need and fulfillment of that need in the present invention. The present invention describes an improvement over prior art dual, spiral, planar antennas which as described above were fabricated by etching copper on both sides of a thin dielectric sheet and positioning inputs to said spirals orthogonally to the spiral plane and centrally positioned with respect to the spiral.
The present invention describes a substantial improvement over prior art spiral antenna technology by providing a parallel (planar) edge connecting capability (balun) as opposed to an orthogonal, centrally mounted balun input. By such means a spiral antenna can be mounted in aircraft or missile components in a manner that heretofore was impossible or at least difficult, cumbersome, and manpower costly.
In addition, since the dual spiral is fabricated by etching copper on both sides of a thin dielectric sheet, and having an input connection on one edge thereof, it should be readily apparent that a plurality of such antennas can be mounted and interconnected on a planar surface in connection with or as an extension of an integrated circuit. In fact, lowpass, highpass, or bandpass filters could be etched into the spiral plate itself eliminating needless discrete component cost in fabrication thereof and for making a very compact unit.
Another advantage of this invention is improved performance of the antenna over an extended frequency range by close coupling and limited interference from the input junction (balun). By providing the balun on the outer wing of the spiral where longer wavelengths and lower frequencies are associated, balun interference is substantially curtailed as opposed to prior art where the balun was centrally mounted on the spiral and where smaller wavelengths and higher frequencies are associated.
In addition, the present invention describes a general spiral (r=f(.phi.)) that can be implemented in addition to prior art log spirals (r=e.sup.a.phi.) or Archimedean spirals (r=a.phi.).
A further improvement in the present invention over the prior art is that a stripline configuration is implemented wherein the conductor spiral of the prior art microstrip orientation in addition to using the second radiating spiral as a ground plane is provided with a counter-opposed radiating spiral, ground plane on the opposite surface of the conductor thereby providing a double ground plane and thereby effectively eliminating any stray interference from the electric field therebetween.
Generally, prior art devices are substantially more cumbersome, labor intensive, and limited in bandwidth capability as well as systems applications. The present invention eliminates these shortcomings and further improves the reliability and quality of spiral antennas in general.